6 THÉEL, DEVELOPMENT OF ECHINUS MILIARIS L. 



in contact witli the pentagonal floor of the amniotic cavity, 

 has become transformed into a liollow ring open posteriorly 

 and provided witli five liollow processes which constitnte the 

 first traces of »radial canals» of the futnre Echiniis. Låter on, 

 the ring becomes elosed posteriorly. With regard to the 

 formation of the hydrocoele in Echinus esculentns MacBride 

 writes: »The latter has now taken on the form of a flattened 

 pentagon. It never has the form of an open hoop, so charac- 

 teristic of the corresponding organ in Asteroidea, Ophiuroidea 

 and flolothnroidea.^> Farther on he says: »The first trace of 

 the adnlt mouth and oesophagus appears as an invagination 

 of the oral disc, indenting the centre of the hydrocoele. At 

 a låter period it completely perforates the hydrocoele, thus 

 converting it from a flattened plate into a ring.» As has 

 been shewn above, this is not the case in Echinus miliaris. 

 The terminations of the five projections from the hydro- 

 coele-ring grow larger, forcing the floor of the primary 

 amniotic cavity to thrust out into this cavity five rounded 

 protuberances which cover the former and form, together 

 ■vvith them, the first trace of the »five primary tube-feeU. These 

 increase rapidly in the same proportion as the amniotic cavity 

 enlarges. Almost at the same period the ectodermic floor of 

 the cavity just mentioned protrudes into the cavity tive 

 interradial folds which alternate with the tube-feet; they 

 assume the shape of small bladders, become flattened, and 

 rapidly extend their margins in order to meet one another 

 and, finally, unite thus forming a double membrane. In this 

 way the amniotic cavity becomes divided into two cavities; 

 the outer and larger of these lodges the tube-feet and may 

 properly retain the name >->amniotic cavity», while the inner, 

 smaller one constitutes the first trace of the »huccal cavity» in 

 the future Echinus and has been called »epineural space» by 

 MacBride. At each of the bases of the five primary tube-feet 

 the epineural space or, rather, buccal cavity, presents a small 

 prolongation which stånds for a time in free communication 

 with the amniotic cavity; these prolongations are the first 

 traces of »epineural canals» and are formed by the five almost 

 flattened bladders overarching the bases of the feet in order 

 to unite and form the double membrane mentioned above. 

 Thus, the buccal cavity communicates for some time by five 

 pores with the amniotic cavity, which in its turn opens 



